Ultracapacitors Boost Performance and Lower Cost

A family of ultracapacitor cells and multicell modules aimed at transportation and industrial applications provides higher energy density and power density, while lowering ultracapacitor costs and increasing cycle life. The new cells and modules from Maxwell Technologies (San Diego, Calif.) exploit proprietary improvements in ultracapacitor electrode composition, a mechanical design that closely matches the electrode with the cell package and the ability to minimize impurities introduced into the cell during manufacturing.

A continuation of the BOOSTCAP series, the MC2600 is a 2600-F cell engineered for operation at 2.7 V rather than the typical 2.5 V. The BMOD2600-16 module combines six of the MC2600 cells to provide 16-V operation. The MC2600 achieves a power density of 10.4 kW/kg and an energy density of 5.6 Wh/kg.

At the same time, the cell continues the company's efforts to reduce ultracapacitor cost, which is the critical barrier to ultracapitor use in many potential applications. The new cell is being introduced at $27 per unit in annual volumes of one million cells, which translates to roughly a penny per Farad. Maxwell Technologies plans to cut this cost in half within five years.

To achieve lower cost, while raising cell performance, the company improved its electrode technology. The challenge here was to develop the right formulation of carbon material and to engineer a process for coating this material — in powder form — onto an aluminum foil. Besides being cost effective in production, the process must ensure that the carbon material does not delaminate from the foil in the presence of the electrolyte that's used in the cell. In addition, the mechanical design of the package was simplified by cutting the parts count and reducing the number of steps to manufacture the cell.

Beyond the cost of the individual cell, the MC2600 enables savings at the system level because its higher operating voltage means that fewer cells are required in the target application. Typically, multiple cells must be strung together. For example, one hybrid bus application currently strings together about 288 (2.5-V) ultracapacitor cells to achieve 700-V energy storage. In applications such as this one, the 2.7-V rating on the MC2600 will reduce the number of cells required.

Although 2.5 V is the operating voltage commonly specified for ultracapacitors, the theoretical voltage limit for these devices is around 3.5 V. The purity of the electrode material and techniques used to seal the electrode in the ultracapacitor package determine how close you can get to the theoretical limit when specifying the ultrcap's operating voltage. These same factors also influence cell life, and the company leveraged its improvements in electrode composition and cell packaging to extend the cell life from the 500,000 cycles previously specified to more than one million cycles.

The cylindrical MC2600 cells measure 138 mm × 57.7 mm and weigh 470 g. They feature a double-ended design available in two versions that facilitate either mechanical or welded termination. They are priced at $92 per cell in low volume and $54 per cell in mid-range volume.